private volatile int state

public class LockExample {

    static int count = 0;
    static ReentrantLock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {

        Runnable runnable = new Runnable() {
            @Override
            public void run() {

                try {
                    // 加鎖
                    lock.lock();
                    for (int i = 0; i < 10000; i++) {
                        count++;
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
                finally {
                    // 解鎖，放在finally子句中，保證鎖的釋放
                    lock.unlock();
                }
            }
        };

        Thread thread1 = new Thread(runnable);
        Thread thread2 = new Thread(runnable);
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        System.out.println("count: " + count);
    }
}

/**
 * 輸出
 * count: 20000
 */

// 樂(lè)觀鎖 + 狀態(tài)機(jī)
update
    table_name
set
    version = version + 1,
    count = count + 1
where
    id = id AND version = version AND count = [修改前的count值];

// 行鎖 + 狀態(tài)機(jī)
 update
    table_name
set
    count = count + 1
where
    id = id AND count = [修改前的count值]
for update;

public class LockExample {
    private static Map<String, Lock> lockMap = new ConcurrentHashMap<>();
    
    public static void lock(String userId) {
        // Map中添加細(xì)粒度的鎖資源
        lockMap.putIfAbsent(userId, new ReentrantLock());
        // 從容器中拿鎖并實(shí)現(xiàn)加鎖
        lockMap.get(userId).lock();
    }
    public static void unlock(String userId) {
        // 先從容器中拿鎖，確保鎖的存在
        Lock locak = lockMap.get(userId);
        // 釋放鎖
        lock.unlock();
    }
}

public class LockExample {
    public static void syncFunc1(Long accountId) {
        String lock = new String(accountId + "").intern();

        synchronized (lock) {

            System.out.println(Thread.currentThread().getName() + "拿到鎖了");
            // 模擬業(yè)務(wù)耗時(shí)
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }

            System.out.println(Thread.currentThread().getName() + "釋放鎖了");
        }
    }

    public static void syncFunc2(Long accountId) {
        String lock = new String(accountId + "").intern();

        synchronized (lock) {

            System.out.println(Thread.currentThread().getName() + "拿到鎖了");
            // 模擬業(yè)務(wù)耗時(shí)
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }

            System.out.println(Thread.currentThread().getName() + "釋放鎖了");
        }
    }

    // 使用 Synchronized 來(lái)實(shí)現(xiàn)更加細(xì)粒度的鎖
    public static void main(String[] args) {
        new Thread(()-> syncFunc1(123456L), "Thread-1").start();
        new Thread(()-> syncFunc2(123456L), "Thread-2").start();
    }
}

/**
 * 打印
 * Thread-1拿到鎖了
 * Thread-1釋放鎖了
 * Thread-2拿到鎖了
 * Thread-2釋放鎖了
 */

// 扣庫(kù)存接口
@RequestMapping("/minusInventory")
public String minusInventory(Inventory inventory) {
    // 獲取鎖
    String lockKey = "lock-" + inventory.getInventoryId();
    int timeOut = 100;
    Boolean flag = stringRedisTemplate.opsForValue()
            .setIfAbsent(lockKey, "竹子-熊貓",timeOut,TimeUnit.SECONDS);
    // 加上過(guò)期時(shí)間，可以保證死鎖也會(huì)在一定時(shí)間內(nèi)釋放鎖
    stringRedisTemplate.expire(lockKey,timeOut,TimeUnit.SECONDS);
    
    if(!flag){
        // 非阻塞式實(shí)現(xiàn)
        return "服務(wù)器繁忙...請(qǐng)稍后重試！??！";
    }
    
    // ----只有獲取鎖成功才能執(zhí)行下述的減庫(kù)存業(yè)務(wù)----        
    try{
        // 查詢庫(kù)存信息
        Inventory inventoryResult =
            inventoryService.selectByPrimaryKey(inventory.getInventoryId());
        
        if (inventoryResult.getShopCount() <= 0) {
            return "庫(kù)存不足，請(qǐng)聯(lián)系賣家....";
        }
        
        // 扣減庫(kù)存
        inventoryResult.setShopCount(inventoryResult.getShopCount() - 1);
        int n = inventoryService.updateByPrimaryKeySelective(inventoryResult);
    } catch (Exception e) { // 確保業(yè)務(wù)出現(xiàn)異常也可以釋放鎖，避免死鎖
        // 釋放鎖
        stringRedisTemplate.delete(lockKey);
    }
    
    if (n > 0)
        return "端口-" + port + ",庫(kù)存扣減成功?。?！";
    return "端口-" + port + ",庫(kù)存扣減失敗?。?！";
}

作者：竹子愛(ài)熊貓
鏈接：https://juejin.cn/post/7038473714970656775

// 續(xù)命子線程
public class GuardThread extends Thread { 
    private static boolean flag = true;

    public GuardThread(String lockKey, 
        int timeOut, StringRedisTemplate stringRedisTemplate){
        ……
    }

    @Override
    public void run() {
        // 開(kāi)啟循環(huán)續(xù)命
        while (flag){
            try {
                // 先休眠一半的時(shí)間
                Thread.sleep(timeOut / 2 * 1000);
            }catch (Exception e){
                e.printStackTrace();
            }
            // 時(shí)間過(guò)了一半之后再去續(xù)命
            // 先查看key是否過(guò)期
            Long expire = stringRedisTemplate.getExpire(
                lockKey, TimeUnit.SECONDS);
            // 如果過(guò)期了，代表主線程釋放了鎖
            if (expire <= 0){
                // 停止循環(huán)
                flag = false;
            }
            // 如果還未過(guò)期
            // 再為則續(xù)命一半的時(shí)間
            stringRedisTemplate.expire(lockKey,expire
                + timeOut/2,TimeUnit.SECONDS);
        }
    }
}


// 創(chuàng)建子線程為鎖續(xù)命
GuardThread guardThread = new GuardThread(lockKey,timeOut,stringRedisTemplate);
// 設(shè)置為當(dāng)前 業(yè)務(wù)線程 的守護(hù)線程
guardThread.setDaemon(true);
guardThread.start();

作者：竹子愛(ài)熊貓 
鏈接：https://juejin.cn/post/7038473714970656775